By way of example, the first measurement system can have an optical photomask inspection system or an optical metrology system for photomasks. In such measurement systems, the first imaging optical unit exhibits a distortion of the imaged image field. Consequently, the positions and distances of the imaged image field are erroneous in relation to the positions and distances actually present on the imaged sample. This error, which is purely caused by the first imaging optical unit, can be a multiple of the measurement accuracy to be measured in the image field, even in the case of a modern first measurement system.
Thus, for example, the distortion error of a modern metrology system can be 30 nm on the photomask to be imaged, whereas the measurement accuracy of the metrology system would have to be less than 1 nm in relation to the photomask, for example for measuring marks and distances of marks on the photomask in the image field.
According to the prior art, a conventional method for correcting the distortion of an optical system consists of imaging an ideal, i.e., error-free, test structure with the aid of the optical system and then determining the distortion error by way of the comparison between test structure and image. These can then be used to correct within the meaning of a higher measurement accuracy further images recorded with the aid of the optical system.
However, on account of the extreme requirements on the measurement accuracy, it is very difficult in the present case, i.e., the correction of distortion of optical systems used in microlithography, to manufacture error-free test structures. Here, “error-free” means test structures with deviations of structure positions from the intended value that are significantly lower than the required measurement accuracy of the measurement system. Consequently, a correction of these optical systems according to the prior art is very difficult.